Three carbon-enhanced metal-poor dwarf stars from the SDSS

Behara, N. T.; Bonifacio, P.; Ludwig, H.‐G.; Sbordone, L.; Hernández, J. I. González; Caffau, E.

doi:10.1051/0004-6361/200913213

Cited by 95 publications

(149 citation statements)

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“…6. Similar plots have already been shown in Behara et al (2010), Masseron et al (2010), Spite et al (2013) and Caffau et al (2013a).…”

Section: On the Carbon Abundances In Cemp Starssupporting

confidence: 85%

“…Remarkably we measure also the lithium abundance in this star that places it on the Spite plateau (Spite & Spite 1982). This pattern is indeed similar to that of SDSS J1036+1212 (Behara et al 2010), that is a 3 1 pm = 10 −12 m. …”

Section: Sdss J0212+0137supporting

confidence: 77%

“…The Sr and Ba abundances measured in this star suggest that it should be classified as a CEMP-rs star, using the criterion of Masseron et al (2010). Another remarkable feature of this star is its Li abundance, with A(Li) = 2.26 it lies clearly on the Spite plateau, unlike the majority of CEMP dwarf stars where Li is not measured or appears clearly below the Spite plateau (Sivarani et al 2006;Behara et al 2010). …”

Section: Sdss J1137+2553mentioning

confidence: 79%

“…This implies that NLTE and granulation effects have opposite directions, and they may tend to cancel; however, for iron it is not possible to simply add the 3D Thompson et al 2008;Aoki et al 2008;Behara et al 2010;Placco et al 2011;Carollo et al 2012;Masseron et al 2010Masseron et al , 2012Yong et al 2013;Cohen et al 2013;Spite et al 2013;Caffau et al 2013a). The bimodal distribution is clear in this plot.…”

Section: Effects Of Granulationmentioning

confidence: 93%

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TOPoS

Bonifacio

Caffau

Spite

et al. 2015

A&A

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Context. In the course of the Turn Off Primordial Stars (TOPoS) survey, aimed at discovering the lowest metallicity stars, we have found several carbon-enhanced metal-poor (CEMP) stars. These stars are very common among the stars of extremely low metallicity and provide important clues to the star formation processes. We here present our analysis of six CEMP stars. Aims. We want to provide the most complete chemical inventory for these six stars in order to constrain the nucleosynthesis processes responsible for the abundance patterns. Methods. We analyse both X-Shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D local thermodynamic equilibrium (LTE) model atmospheres and the turbospectrum line formation code. were not able to detect any iron lines, yet we could place a robust (3σ) upper limit of [Fe/H] < −5.0 and measure the Ca abundance, with [Ca/H] = −5.0, and carbon, A(C) = 6.90, suggesting that this star could be even more metal-poor than SDSS J1742+2531. This makes these two stars the seventh and eighth stars known so far with [Fe/H] < −4.5, usually termed ultra-iron-poor (UIP) stars. No lithium is detected in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which implies a robust upper limit of A(Li) < 1.8 for both stars. Conclusions. Our measured carbon abundances confirm the bimodal distribution of carbon in CEMP stars, identifying a high-carbon band and a low-carbon band. We propose an interpretation of this bimodality according to which the stars on the high-carbon band are the result of mass transfer from an AGB companion, while the stars on the low-carbon band are genuine fossil records of a gas Article published by EDP Sciences A28, page 1 of 20 A&A 579, A28 (2015) cloud that has also been enriched by a faint supernova (SN) providing carbon and the lighter elements. The abundance pattern of the UIP stars shows a large star-to-star scatter in the [X/Ca] ratios for all elements up to aluminium (up to 1 dex), but this scatter drops for heavier elements and is at most of the order of a factor of two. We propose that this can be explained if these stars are formed from gas that has been chemically enriched by several SNe, that produce the roughly constant [X/Ca] ratios for the heavier elements, and in some cases the gas has also been polluted by the ejecta of a faint SN that contributes the lighter elements in variable amounts. The absence of lithium in four of the five known unevolved UIP stars can be explained by a dominant role of fragmentation in the formation of these stars. This would result either in a destruction of lithium in the pre-main-sequence phase, through rotational mixing or to a lack of late accretion from a reservoir of fresh gas. The phenomenon should have varying degrees of efficiency.

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“…6. Similar plots have already been shown in Behara et al (2010), Masseron et al (2010), Spite et al (2013) and Caffau et al (2013a).…”

Section: On the Carbon Abundances In Cemp Starssupporting

confidence: 85%

Section: Sdss J0212+0137supporting

confidence: 77%

Section: Sdss J1137+2553mentioning

confidence: 79%

Section: Effects Of Granulationmentioning

confidence: 93%

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TOPoS

Bonifacio

Caffau

Spite

et al. 2015

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“…The situation with understanding sources of the carbon production in the early Galaxy is complicated by a discovery of the Carbon-Enhanced Metal-Poor (CEMP) stars that show very high enhancements in carbon, with [C/Fe] up to 3 dex (see recent papers of Behara et al 2010, Spite et al 2013, Ito et al 2013, Lee et al 2013, Cohen et al 2013, Otsuka & Tajitsu 2013, Kennedy et al 2014, Abate et al 2015, Ramirez-Ruiz et al 2015, Aoki et al 2015, Sluder et al 2015. It is still debated whether their carbon enhancements are inborn, or their atmospheres were polluted, most likely by accretion from an AGB binary companion.…”

Section: Introductionmentioning

confidence: 99%

Carbon abundances of reference late-type stars from 1D analysis of atomic C i and molecular CH lines

Alexeeva

Mashonkina

2015

Mon. Not. R. Astron. Soc.

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A comprehensive model atom was constructed for C I using the most up-to-date atomic data. We evaluated non-local thermodynamical equilibrium (NLTE) line formation for neutral carbon in classical 1D models representing atmospheres of late-type stars, where carbon abundance varies from solar value down to [C/H] = −3. NLTE leads to stronger C I lines compared with their LTE strength and negative NLTE abundance corrections, ∆ NLTE . The deviations from LTE are large for the strong lines in the infrared (IR), with ∆ NLTE = −0.10 dex to −0.45 dex depending on stellar parameters, and they are minor for the weak lines in the visible spectral range, with |∆ NLTE | 0.03 dex. The NLTE abundance corrections were found to be dependent of the carbon abundance in the model. As the first application of the treated model atom, carbon NLTE abundances were determined for the Sun and eight late-type stars with well-determined stellar parameters that cover the −2.56 [Fe/H] −1.02 metallicity range. Consistent abundances from the visible and IR lines were found for the Sun and the most metal-rich star of our sample, when applying a scaling factor of S H = 0.3 to the Drawinian rates of C+H collisions. Carbon abundances were also derived from the molecular CH lines and, for each star, they agree with that from the atomic C I lines. We present the NLTE abundance corrections for lines of C I in the grid of model atmospheres applicable to the carbon-enhanced (CEMP) stars.

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Extremely metal‐poor stars in SDSS fields

et al. 2011

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Some insight on the first generation of stars can be obtained from the chemical composition of their direct descendants, extremely metal-poor stars (EMP), with metallicity less than or equal to 1/1000 of the solar metallicity. Such stars are exceedingly rare, the most successful surveys, for this purpose, have so far provided only about 100 stars with 1/1 000 the solar metallicity and 4 stars with about 1/10 000 of the solar metallicity. The Sloan Digital Sky Survey has the potential to provide a large number of candidates of extremely low metallicity. X-shooter has the unique capability of performing the necessary follow-up spectroscopy providing accurate metallicities and abundance ratios for several elements (Mg, Al, Ca, Ti, Cr, Sr, . . . ) for EMP candidates. We here report on the results for the first two stars observed in the course of our Franco-Italian X-shooter GTO. The two stars were targeted to be of metallicity around -3.0, the analysis of the X-shooter spectra showed them to be of metallicity around -2.0, but with a low α to iron ratio, which explains the underestimate of the metallicity from the SDSS spectra. The efficiency of X-shooter allows an in situ study of the outer halo, for the two stars studied here we estimate distances of 3.9 and 9.1 kpc, these are likely the most distant dwarf stars studied in detail to date.

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Three carbon-enhanced metal-poor dwarf stars from the SDSS

Cited by 95 publications

References 72 publications

TOPoS

TOPoS

Carbon abundances of reference late-type stars from 1D analysis of atomic C i and molecular CH lines

Extremely metal‐poor stars in SDSS fields

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